Hitherto, the plasma cutting machine has been mounted on an XY table or the like and adapted to a method of controlling the torch height in plasma cutting in order to obtain quality cutting, the method being arranged to utilize a fact that "torch height h is in proportion to arc voltage Va" at a predetermined torch movement speed (hereinafter called a "cutting speed") in such a manner that the arc voltage Va isomonitored so that the torch is maintained at its optimum height hc (refer to, for example, DE2706232C3).
Specifically, as shown in FIG. 7, an assumption is made that the arc voltage generated when the torch is positioned at the optimum torch height hc is a target arc voltage Vci (SO), the torch is set at the aforesaid optimum cutting height hc from a base metal (S5), and then the base metal is cut. Furthermore, after the arc voltage Va becomes steady (S6), the arc voltage Va is read (S93), and this arc voltage Va and the aforesaid target arc voltage Vc1 are subjected to a comparison (S10). If Va&gt;Vc1, the torch is lowered (S101). If Va=Vc1, the existing torch height is retained (S102). If Va&lt;Vc1, the torch is moved upwardly (S103). As result, the torch is maintained at its optimum torch height hc.
The aforesaid optimum torch height hc is a value which can be determined predictively by collectively considering various factors such as the thickness and the material of the base metal, the diameter of the nozzle of the torch, the cutting speed, and the torch height. The target arc voltage Vc1 obtained from the optimum height hc has been set into the control system of the subject plasma cutting machine as a fixed value.
Observing the actual cut surface, if the torch height is constant at its optimum, the quality of cutting is also constant (in other words, if the torch height is changed, the quality of cutting deteriorates). With the aforesaid conventional method, the cutting speed is changed at the time of cutting a corner, although an assumption is made that a constant cutting speed is maintained. Accordingly, the optimum torch height is also changed, causing the quality of cutting the corner to deteriorate as described in detail below.
FIG. 8 is a characteristic graph which illustrates the results of measurements of cutting speeds (axis of abscissa) and arc voltages (axis of ordinate) with respect to various torch heights h1 to h5 (h1&lt;h5) . As shown in FIG. 8, an increase in the cutting speed Fa will lower the arc voltage Va (hereinafter called an "in inverse proportion") at each torch height h1 to h5. The reason f or this lies in the fact that an increase in the cutting speed causes the main positive point to come closer to the torch.
In a case where the predicted optimum torch height is h1 and the predicted optimum cutting speed is FL, the arc voltage Va is Vc1 (for convenience in description, an assumption is made that the arc voltage Va=the target arc voltage Vc1) . If the cutting speed is raised (FL.fwdarw.FH), the arc voltage Va is lowered (Vc1 .fwdarw.VL). Since the target arc voltage Vc1 is a fixed value, the aforesaid values are subjected to a comparison, resulting in VL&lt;Vc1. Hence, the torch is raised to height h3. That is, there arises a problem in that the torch height has to be changed because the cutting speed has been changed.
In order to overcome the aforesaid problems experienced with the conventional technology, an object of the present invention is to provide a method of controlling the torch height in plasma cutting which is capable of overcoming the deterioration of cutting quality due to a change in the cutting speed.